Continuous liquid-phase rapid pulping



Feb. 7, 1967 w ss ER 3,303,088

CONTINUOUS LIQUID-PHASE RAPID FULPING Filed April 19, 1963 file fl "l7el 3 IMPREG.

ZONE WD) 2a 7 I, J- 71 30 25 COOKING 26 27 A 6? K2 W ZONE WHITE r 7 RLIQUOR A I 3 33 A 32 Q 17 BLACK LIQUOR TO RECOVERY INVENTOR ADOLF W.GESSNER ATTORNEY United States Patent 3,303,088 CONTINUQUS LIQUID-PHASERAPID PULPING Adolf W. Gessner, Montclair, N.J., assignor to The LummusCompany, New York, N.Y., a corporation of Delaware Filed Apr. 19, 1963,Ser. No. 274,186 Claims. (Cl. 162-19) This invention relates to thedelignification of cellulosic materials to prepare cellulosic fibersand, more particularly, to an improved process and apparatus foreffecting the continuous and rapid digestion of cellulosic materials.

. Prior continuous processes and apparatus for effecting thedelignification of cellulosic material have been directed to thesimulation of the processing steps of batchwise operation in acontinuous manner. Accordingly, such continuous pulping processes andapparatus have been designed to reproduce the time, temperature, andpressure relationships of the processing steps of batch operationutilizing continuous processing techniques. For instance, using kraftliquors, the batch digester, once having reached a temperature in therange of from 338 F. to 356 F., is maintained at such temperature for aperiod of from two to three hours to effect delignification. Continuousprocesses designed for kraft liquor operation retain the cellulosicmaterial being treated at a temperature within such temperature rangefor the same two to threehour period. Savings in utilities, of course,are nevertheless realized through continuous operation.

A recent process for continuously digesting fibrous material has foundcommercial acceptance in the industry. In accordance with this process,cellulosic material is introduced by a screw-type feeder into thedigester wherein the cellulosic material continuously passes downwardlythrough the various zones of the digester. In the upper portion of thedigester, impregnation of the cellulosic material is effected at animpregnation temperature within the range of from 60 C. to 120 C. Thematerial is thereafter heated to a temperature of 140 C. to 180 C. andhigher whereby delignification of the cellulosic material isaccomplished in a section below the impregnation section of thedigester. Since a requirement of the process is to effect digestion inthe liquid phase and at a pressure whereby no vaporization of thecooking liquor is effected, a compression liquid is introduced into thelower portion of the digester to maintain the pressure within thedigester above the pressure at which vaporization of the digestionliquor will occur. The digested cellulosic material, together with spentcooking liquor and compression liquor are withdrawn at a temperature of120 C. from the lower portion of the digester and passed to subsequentunits for further treating. In accordance with this process, thecellulosic material is maintained at delignifying temperatures in thedigestion zone of the digester for a period of time normally requiredfor the delignification of cellulosic materials in a batch digester,i.e., two to three hours.

No liquid-phase delignification process has been developed whichsubstantially reduces the time period required for effecting thedelignification of the cellulosic material while improving ormaintaining the quality of the resultant pulp, as well as substantiallyreducing the size of the digester and related equipment for any desiredcapacity.

A primary object of my invention is to efiiect the continuousliquid-phase delignification of cellulosic materials at temperatures andpressures normally employed in batch operations in time periods whichare substantially less than the time periods heretofore required toeffect the delignification of cellulosic material either in batch orcontinuous operation.

Another object of my invention is to provide a continuous process forrapidly effecting the delignification of cellulosic materials whereinthe cellulosic material is rapidly heated to impregnation temperature bypartially spent cooking liquor.

A further object of my invention is to provide a com tinuous process foreffecting the delignification of cellulosic material by rapidly heatingthe impregnated cellulosic material to delignifying temperatures.

Still another object of my invention is to provide a continuous processfor effecting the delignification of cellulosic material having reducedutility requirements.

A still further object of my invention is to provide a continuousprocess for effecting the delignification of cellulosic material inprocessing equipment substantially smaller than equipment heretoforeutilized.

Another object of my invention is to provide a continuous process foreffecting the delignification of cellulosic material wherein thedelignification reaction is rapidly halted in a cooling zone of theapparatus and the delignified material is withdrawn from said zone at atemperature below the boiling point of the cooking liquor at atmosphericpressure whereby cellulosic fiber damage is essentially minimized.

A further object of my invention is to provide a continuous process foreffecting the digestion of cellulosic material in a digestion vesselhaving a cooling zone comprised of at least two sub-sections whereby theliquid flow through each sub-secti0n is concurrent with the flow of thematerial, but the liquor flow between each sub-section iscountercurrent.

Other objects and a fuller understanding of my invention may be had byreferring to the following description taken in conjunction with theaccompanying drawing in which the figure is a partial schematic flowdiagram illustrating a preferred embodiment of my invention foreffecting the rapid and continuous digestion of cellulosic material inliquid phase for the preparation of cellulosic fibers of high strengthand low bleachability requirements.

In accordance with my invention, cellulosic material, after steaming, isintroduced into a feed tank wherein the steamed cellulosic material issubmerged in a heated cooking liquor. A slurry of the cellulosicmaterial and liquor is introduced into the upper portion of a digesterhaving an impregnation zone, a digestion zone, and a cooling zone. Amajor portion of the liquor utilized for carrying the cellulosicmaterial to the upper portion of the digestion zone is withdrawn fromsuch upper portion and returned to the feed tank. The consistency of theslurry introduced into the upper portion of the digestion zone isincreased by the withdrawal of such recycle liquor.

The chip-liquor slurry is contacted with partially spent cooking liquorat a temperature of from about 360 F. to about 385 F. to rapidly raisethe temperature of the chip-liquor slurry to about 240 F. to about 280F. The cellulosic material is passed through the impregnation zone for atime period of about 10 to 35 minutes, preferably from about 20 to about30 minutes, whereby the cellulosic material is substantiallyimpregnated. Thereafter, the chip-liquor slurry is contacted withanother stream of cooking liquor heated to a temperature of from about370 F. to about 400 F. to rapidly raise the temperature of thecellulosic material to a temperature within the range of from about 330F. to about 385 F. The cellulosic material is passed downwardly throughthe digestion zone of the digester for a time period of about 10 to 35minutes, preferably from about 20 to about 25 minutes, to effectdelignification of the cellulosic material. The solids consistency ofthe chip-liquor slurry in both the impregnation zone and the top of thecooking zones is maintained substantially equal and preferably withinthe range of from about 10% to about 20% by weight. Variations ofconsistency of not more than 3% are preferred.

The delignified material or cellulosic fibers passing downwardly fromthe digestion zone of the digester has a solids consistency of about toabout by weight, as a result of the solubilization of approximatelyonehalf of the solids introduced into the digester. A portion of theliquor is withdrawn from the material to increase the solids consistencyto about 10% to about 20% by weight. The delignified material is nowpassed through a cooling zone to reduce the temperature of thedelignified material to a temperature below the atmospheric boilingpoint of the cooking liquor, normally about 212 F. to about 220 F.thereby substantially eliminating fibre damage due to steam flashingduring subsequent pressure reduction. The delignified material issubsequently passed to a screen to remove knots and undigestedcellulosic material, and thereafter washed to remove substantially allof the spent cooking liquor and then passed to subsequent units forfurther processing.

The cooling zone of the digester is'comprised of at least twosub-sections. The flow of the delignified material and liquor throughsuch sub-sections is concurrent; however, the sequence of contactbetween the cooling liquor and material to be cooled in suchsub-sections is in effect countercurrent. The cooling liquor is firstintroduced into the lower cooling sub-section and contacts thedelignified material which has been partially cooled in the uppercooling sub-section. The cooling liquor is then withdrawn from the lowercooling sub-section and is introduced into the upper cooling sub-sectionfrom which the liquor is withdrawn after partially cooling the materialpassing into the upper cooling sub-section from the delignificationzone. The object of this countercurrent flow arrangement is to cool thedelignified material below the atmospheric boiling point of the liquorwith a substantially smaller quantity of cooling liquor than would berequired with concurrent flow. The object of maintaining concurrent flowwithin each individual cooling sub-section is to avoid the difficultiesassociated with countereurrent flow of liquor and delignified material,i.e., excessive fluid pressure drop resulting in the compression of thedelignified solids, which may cause even higher fluid pressure drops andeventual plugging of the equipment, or extremely low velocities of theliquor relative to the solids which necessitate uneconomically largevessels for a given throughput of materials.

As will be readily apparent, the combined impregnation-delignificationtime period in accordance with the present process is significantly lessthan impregnationdelignification time periods of processes presentlypracticed, whether batch or continuous.

Now referring to the drawing, wood chips or like cellulosic materialprepared in a normal manner and at ambient temperatures are introducedthrough line 1 into storage bin 2. The wood chips are withdrawn fromstorage bin 2 through line 3 controlled by rotary valve 4, and passedinto a screw conveyor 5 driven by motor 6. The wood chips are preheatedas they are passed through the screw conveyor 5 by low pressure steamintroduced into screw conveyor 5 through line 7. Steam condensate iswithdrawn from the screw conveyor 5 through line 8. The thus preheatedand steamed chips are passed from screw conveyor 5 through line 9,controlled by rotary valve 10, and are introduced into feed tank 11. Thepreheated chips in feed tank 11 are submerged in a liquor formed ofcooking liquor and recycled liquor introduced into the feed tank 11through lines 12 and 13, respectively.

A slurry of wood chips and liquor is withdrawn from the storage tank 11through line 14 by pump 15 and passed through line 16 into the upperportion of a digester, generally indicated as 17. The consistency of thechipliquor slurry introduced into the digester 17 is from about 1% to6%, preferab y about 4%, and is determined y 5% to about 25%, preferably10% to 20%.

4 various factors, such as the size of the chips, temperature andpressure of digestion, the construction of pump 15, etc. Below the pointat which the slurry is introduced, a major portion of the liquor iswithdrawn through a strainer screen 18, or other suitable devices knownin the art, and is passed through line 13 to the feed tank 11, therebyincreasing the consistency of the slurry.

Immediately below the screen 18, partially spent cooking liquor at atemperature of about 350 F. to about 385 F. in line 19 is introducedthrough a plurality of nozzles, generally indicated as 20, into thechip-liquor slurry. The velocity at which the partially spent cookingliquor is introduced into the chip-liquor slurry is sufiicient to effectturbulent mixing of the slurry and partially spent cooking liquor andthereby rapidly heat the chipliquor slurry to a temperature betweenabout 240 F. to about 280 F. Thus, the cellulosic material is heated toimpregnation temperatures within from about 10 to about 30 seconds.Excess liquor is withdrawn from a strainer screen 21 through line 22 tomaintain the consistency of the slurry passing through the impregnationzone at about The chipliquor slurry is caused to pass through theimpregnation zone for a period preferably of from about 20 to about 30minutes to effect substantial impregnation of the cellulosic material.

At the lower portion of the impregnation zone, hot cooking liquor inline 23 is introduced through a plurality of nozzles, generallyindicated as 24, into the chip-liquor slurry passing from theimpregnation zone. The cooking liquor is heated to a temperature of fromabout 360 F. to about 400 F. and is passed through the nozzles 24 at avelocity such that turbulent mixing of the chips and heated cookingliquor effects rapid heating of the chips to a temperature of from 330F. to 385 F. A strainer screen 25 is positioned below the nozzles 24 andexcess liquor is withdrawn through line 26 by pump 27 and is heated inheat exchanger 28 to form the heated cooking liquor introduced into thechip-liquor slurry through line 23. Steam in line 29 is introduced intothe heat exchanger 28 to heat the cooking liquor, with steam condensatebeing withdrawn therefrom through line 30. The consistency of thechip-liquor slurry passing into the digestion zone of the digester 17 issubstantially the same as that of the chip-liquor slurry passing throughthe impregnation zone. The consistency of the slurry in the digestionzone is maintained at about 5% to about 25 preferably from 10% to 20%.The consistencies of the slurries passing through the impregnation anddigestion zones may be independently controlled by recirculation ofliquor through lines (not shown), but are usually maintained at aboutthe same percentage. In some instances, the consistencies may varydepending on the species of cellulosic material, temperature ofimpregnation and delignification and utility requirement considerations.

The now heated chip-liquor slurry is caused to pass through thedigestion zone of the digester 17 for a time period of about from 15 toabout 30 minutes, preferably from 15 to about 30 minutes, preferablyfrom 20 to 25 minutes, to effect solubilization of the lignins in thecellulosic material to produce cellulosic fibers. Partially spentcooking liquor is withdrawn through a strainer screen 31 positioned atthe lower portion of the digestion zone. A portion of the liquorwithdrawn from the screen 31 is passed through line 32 by pump 33 andconstitutes the partially spent cooking liquor introduced into thechip-liquor slurry in line 19 to elfect the rapid heating of the woodchips to the impregnation temperature thereof. The remaining portion ofthe partially spent cooking liquor withdrawn through screen 31 is passedthrough line 32a controlled by valve 33a into a flash drum 34. Theliquor is flashed in the drum 34 and forms the steam which is introducedinto the screw conveyor 5 through line 7 controlled by valve 35.

Below the screen 31, a cooled and partially spent liquor stream in line36 is introduced into the digester 17 through a plurality of nozzles,generally indicated as 37, to effect a rapid and partial cooling of thenow delignified material, as well as a partial washing of the spentcooking liquor from such material. The nozzles 37 function in a mannersimilar to the nozzles 20 and 24 in that turbulent mixing is effected torapidly cool the delignified material. Excess liquor is withdrawnthrough a strainer screen 38 and passed through line 39 for furtherprocessing as hereinafter described.

Below screen 38, a second cooling liquor stream in line 40 is introducedthrough a plurality of nozzles, generally indicated as 41, into thepartially cooled material to effect further cooling of the material, aswell as a further washing thereof. The material is thus cooled to atemperature below or just slightly above the boiling point of the liquorat atmospheric pressure. Excess liquor is withdrawn from the lowerportion of the cooling zone of the digester 17 through strainer screen42 and is passed through line 43 by pump 44 and constitutes the firstcooling liquor stream in line 36 introduced through the nozzles 37 intomaterial passing through the digester 17.

In the event that the temperature of the delignified material at theexit of the lower cooling sub-section is slightly above the atmosphericboiling point of the liquor, or that the slurry consistency at thispoint is too high for smooth discharge through the pressure reducingvalve 48, further cooling liquor may be introduced through line 45 and aplurality of nozzles, generally indicated as 46, to cool the material toa temperature below the atmospheric boiling point of the liquor and/ orto reduce the slurry consistency to a level at which the flow throughthe reducing valve 48 is smooth and mechanical fibre damage minimized.Thus, the consistency of the delignified material withdrawn from thedigester is normally of from about 3% to about 15 preferably from 5% toThe quantity of liquor introduced into the downwardly flowing materialin digester 17 through the nozzles 37 and 41 is substantially equal tothe quantity of liquor withdrawn through the screens 38 and 42.

The now cooled delignified material is withdrawn through line 47,controlled by reducing valve 48. Since the temperature of thedelignified material and liquor withdrawn from the digester is below theboiling point of the liquor at atmospheric pressure, there is noflashing of steam as the material and liquor pass through the reducingvalve 48 and, consequently, fibre damage is minimized. The materialpassing through the valve 48 is passed through line 49 and contactedwith a wash water stream in line 50. The combined stream is passedthrough line 51 to a screen 52 wherein the cellulosic fibers are freedof knots and undissolved cellulosic material. The wash water stream inline 50 is introduced into the delignified material to reduce the solidsconsistency and thereby to facilitate the removal of knots andundissolved cellulosic material. Knots and undissolved cellulosicmaterial are withdrawn from the screen 52 through line 53. Thedelignified material is withdrawn from the screen 52 through line 54 andis passed to a rotary vacuum washer 55. In the rotary vacuum washer 55,wash water is introduced through line 56 to leach out spent cookingliquor from the delignified material or cellulosic fibres. Washed pulpis withdrawn from rotary vacuum wash 55 through line 57 and is passed tosubsequent units (not shown) for further processing.

The wash water from washer 55 is withdrawn through line 58 and isseparated into two portions in lines 59 and 60. The portion in line 59is passed by pump 61 and introduced through line 50 into the delignifiedmaterial in line 49. The portion in line 60 is passed by pump 62 andintroduced into the digester 17 through line 40 and constitutes theaforementioned second cooling stream. A portion of the Wash water inline 40 may be introduced into the digester through line 45.

Partially spent cooking liquor is withdrawn from flash drum 34 throughline 63. The liquor streams in lines 22 and 63, together with a portionof the black liquor in line 39 are combined and passed through line 64to be combined with the fresh cooking liquor or white liquor in line 65for introduction into the storage tank 11 through line 12. The remainingportion of the black liquor in line 39 is passed through line 66 to ablack liquor recovery unit (not shown) for treatment of the black liquorto recover the chemical values thereof.

It is apparent from the aforementioned description of the two-stagecooling zone that the liquor and delignified material is passedconcurrently through the individual sections of the cooling zone of thedigester 17. Countercurrent flow of delignified material and liquor isavoided since at even low relative velocities of delignified material toliquor, high fluid pressure gradients may be formed which lead toexcessive compression of the material and possible plugging of theequipment. It is noted, however, that the two sections of the coolingzone are designed to provide for a countercurrent sequence of contact ofthe cooling liquor with the delignified material while providing forconcurrent flow of material and liquor within each cooling subsection.In this manner, the delignified material may be rapidly cooled to atemperature below the boiling point of the cooking liquor prior to beingwithdrawn from the digester 17 through line 47.

The countercurrent flow of the liquor between the two stages of thecooling zone effects a substantial washing of the delignified materialbefore the material is withdrawn from the digester 17. Consequently, thecellulosic fibres passed to the rotary vacuum washer 55 through line 54contain a substantially smaller quantity of black liquor solids whichpermit final washing of the cellulosic fibres in fewer stages comparedto pulp prepared in known batch or continuous processes.

As an example of my invention, on an hourly flow basis, 3600 pounds ofwood chips (oven-dry basis) and 2400 pounds of moisture are introducedinto the chip bin 2 through line 1, and passed via line 3 into rotaryscrew 5. Steam at 15 p.s.i.g. is introduced into the screw conveyor 5through line 7. Preheated chips are withdrawn from the screw conveyor 5and introduced into feed tank 11 at about 250 F. 6000 pounds of alkalinewhite liquor in line 65 at a temperature of 70 F., together with acombined liquor stream in line 64, of 12,000 pounds and the recycleliquor in line 13 of 48,000 pounds are continuously introduced into thefeed tank 11. The temperature of the chip-liquor slurry in the tank 11is maintained at a temperature of 250 F.

72,000 pounds of chip-liquor slurry at a consistency of 5% are withdrawnthrough line 14 by pump 15 and introduced into the digester 17 throughline 16. 48,000 pounds of the liquor are withdrawn through screen 18 andreturned to the feed tank 11 through line 13. 3400 pounds of partiallyspent cooking liquor in line 19 at a temperature of 360 F. areintroduced into the chipliquor slurry through the nozzles 20 below thescreen 18 and rapidly raise the temperature of the slurry to 265 F.

3400 pounds of cooking liquor are withdrawn through screen 21 to formchip-liquor slurry having a consistency of 15%. The slurry is caused topass through the impregnation zone at a flow rate whereby the chips aresubstantially impregnated after 20 minutes. Above the screen 25, 101,000pounds of a cooking liquor withdrawn from screen 25 through line 26 andheated in heat exchanger 28 to a temperature of 380 F. are introducedthrough nozzles 24 into the chip-liquor slurry to rapidly raise thetemperature of the cellulosic material to the delignificationtemperature of 360 F. The slurry is maintained at a consistency of 15%as it passes through the digestion zone of the digester 17. The chipsare passed through the zone at a flow rate whereby the chips aresubstantially delignified after 20 minutes. 12,000

pounds of partially spent cooking liquor are withdrawn from screen 31 ata temperature of 360 F. 3400 pounds of the liquor withdrawn from screen31 are passed through line 32 by pump 33 and introduced into thechipliquor slurry through nozzles 20. 8600 pounds of the liquorwithdrawn from screen 31 are passed through line 32 to flash drum 34wherein the pressure is reduced to about 35 p.s.i.g. 665 pounds of steamat a temperature of about 280 F. are passed to the screw conveyorthrough line 7 and valve 35. The liquor withdra'wnfrom flash drum 34through line 63 at a temperature of about 280 F. is passed through lines64 and 12 to the feed tank 11.

15,600 poundsof liquor at a temperature of 209 F. are introduced throughthe nozzles 37 into the chipliquor slurry and decrease the consistencyof the material and reduce the temperature of the material to atemperature of about 267 F. by the time the material reaches the screen38. 15,600 pounds of liquor at a temperature of 267 F. are withdrawnfrom screen 38 and passed through line 39. A minor portion of the blackliquor is returned via lines 64 and 12 to feed tank 11 and the remainingportion of the black liquor is withdrawn through line 66 for recovery ofcooking chemicals.

The partially cooled delignified material is further contactedimmediately below the screen 38 with 15,600 pounds of an additionalcooling liquor in line 40 having a temperature of 170 F. The material isnow rapidly cooled to a temperature of 209 F. by the time it reaches thescreen 42. 15,600 pounds of liquor at a temperature of 209 F. arewithdrawn through screen 42 and line 43 and passed by pump 44 throughline 36 and are introduced into the material through nozzles 37 locatedbelow the screen 31, as described above.

The consistency of the chip-liquor slurry is decreased to by injectionof 5000 pounds of liquor through line 45 to permit effective flow of thedelignified material and liquor through the reduction valve 48. Thiscools the material to about 196 F. The delignified material and cookingliquor in line 48 are contacted with a wash liquor in line 50 having atemperature of 170 F., the combined stream being passed to screen 52 toremove knots and undissolved cellulosic material. The delignifiedmaterial is then passed to rotary vacuum washer unit 55 wherein it iscontacted with wash water in line 56 having a temperature of 150 F. 1800pounds of washed pulp or cellulosic fibres (oven-dry basis) arethereupon withdrawn from the rotary vacuum washer unit 55 at line 57 andare passed to subsequent processing units (not shown).

While a prefered embodiment of my invention has been illustarted anddescribed, variations thereof may be made by one skilled in the art,and, therefore, the invention as disclosed hereinabove is intended to belimited only by the scope of the disclosure and the dependent claims.

I claim:

1. A process for preparing pulp by the continuous delignification ofcellulosic material at superatmospheric pressure in a digester having animpregnation zone, a delignification zone and a cooling zone, whichcomprises:

introducing a preheated slurry of cellulosic material and cooking liquorinto the upper portion of said digester;

rapidly heating said slurry to a temperature of from about 240 F. toabout 280 F.; effecting impregnation of said cellulosic material in saidimpregnation zone of said digester;

rapidly heating said impregnated material to a temperature of from about330 F. to about 385 F.; effecting delignification of said cellulosicmaterial in said delignification zone of said digester;

cooling said delignified material in a cooling zone comprised of atleast two sub-stages in said digester; and withdrawing said delignifiedmaterial from said digestion zone.

2. The process of claim 1 wherein the cellulosic material is maintainedin the impregnation zone for about 20 to about 30 minutes.

3. The process of claim 2 wherein the cellulosic material is maintainedin the delignification Zone for about 20 to about 25 minutes.

4. The process as defined in claim 3, wherein the consistency of thecellulosic material and cooking liquor passing through said impregnationzone, delignification zone and cooling zone is from about 10% to about20%.

5. The process of claim 4 wherein partially spent cooking liquor iswithdrawn from a lower portion of said delignification zone and isintroduced into an upper portion of said impregnation zone to effect therapid heating of said slurry.

6. A process for preparing pulp by the continuous delignification ofcellulosic material at superatmospheric pressure in a digester having animpregnation zone, a delignification zone and .a cooling zone, whichcomprises:

introducing a preheated slurry of cellulosic material and cooking liquorinto the upper portion of said digester;

rapidly heating said slurry to a temperature of from about 240 F. toabout 280 F.;

effecting impregnation of said cellulosic material in said impregnationzone of said digester; rapidly heating said impregnated material to atemper ature of from about 330 F. to about 385 F.;

etfecting delignification of said cellulosic material in saiddelignification zone of said digester;

introducing a first cooling liquor stream into the upper portion of saidcooling zone to partially cool said material and to leach out spentcooking liquor;

withdrawing a portion of the liquor stream from a point intermediate ofsaid upper and lower portions of said cooling zone;

introducing a second cooling liquor stream into said delignifiedmaterial below said intermediate point to further cool said material;

withdrawing a portion of the liquor stream from a lower portion of saidcooling zone and passing said liquor stream to the upper portion-of saidcooling zone as said first cooling liquor stream;

and withdrawing delignified material from said digester below saidcooling zone.

7. The process as defined in claim 6, wherein said cellulosic materialis held in said impregnation zone for about 20 to about 30 minutes.

8. The process as defined in claim 7, wherein said cellulosic materialis held in said delignification zone for about 20 to about 25 minutes.

'9. The process as defined in claim 8, and additionally comprisingcontacting said cellulosic material with steam to preheat said materialand forming a slurry of about 1% to about 6% with cooking liquor priorto introducing said slurry into said digester.

10. A process as defined in claim 8 wherein partially spent cookingliquor is withdrawn from the lower portion of said delignification zoneand is introduced into the upper portion of said impregnation zone toeffect the rapid heating of said slurry.

11. A process as defined in claim 8 wherein a portion of the blackliquor withdrawn from said intermediate point of said cooling zone ispassed to a recovery unit.

12. A process as defined in claim 10 wherein a portion of the liquorwithdrawn from the lower portion of said delignification zone is passedto a flash zone and the steam formed therein is used to preheat saidcellulosic mate rial.

13. A process as defined in claim 8 wherein the delignified materialwithdrawn from said digester is washed with water and a portion of saidwash water is passed to an intermediate point in said cooling zone ofsaid digester as said second cooling liquor stream.

14. The process of claim 8 wherein the solids consist- 9 10 ency in eachof said zones is maintained at a value between withdrawing a portion ofthe liquid from a lower porabout 10% and about 20%. tion of said coolingzone and passing said portion of 15. In a process for preparing pulp bythe continuous liquid withdrawn from the lower portion to the upperdelignification of cellulosic material wherein the celluportion of saidcooling zone as said first cooling liquid losic material is successivelypassed through an impregna- 5 stream; and tion zone, a delignificationzone, and a cooling zone, an Withdrawing delignified material from saiddigester beimprovement comprising: law said cooling zone.

introducing a first cooling liquid stream into an upper portion of saidcooling zone to partially cool the References Cited y the Examine!delignified cellulosic material and to leach out spent 10 UNITED STATESPATENTS hquor 3,097,987 7/1963 Solman 16217 Withdrawing a portion of theliquid from an intermediate portion of said cooling zone; introducing asecond cooling liquid stream below the DONALL H SYLVESTER PrimaryExaminer intermediate portion of said cooling zone to further 15 coolthe material; HOWARD R. CAINE, Examiner.

3,200,032 8/1965 Richter et a1. 162-19

1. A PROCESS FOR PREPARING PULP BY THE CONTINUOUS DELIGNIFICATION OFCELLULOSIC MATERIAL AT SUPERATMOSPHERIC PRESSURE IN A DIGESTER HAVING ANIMPREGNATION ZONE, A DELIGNIFICATION ZONE AND A COOLING ZONE, WHICHCOMPRISES: INTRODUCING A PREHEATED SLURRY OF CELLULOSIC MATERIAL ANDCOOKING LIQUOR INTO THE UPPER PORTION OF SAID DIGESTER; RAPIDLY HEATINGSAID SLURRY TO A TEMPERATURE OF FROM ABOUT 240*F. TO ABOUT 280*F.;EFFECTING IMPREGNATION OF SAID CELLULOSIC MATERIAL IN SAID IMPREGNATIONZONE OF SAID DIGESTER; RAPIDLY HEATING SAID IMPREGNATED MATERIAL TO ATEMPERATURE OF FROM ABOUT 330*F. TO ABOUT 385*F.; EFFECTINGDELIGINIFICATION OF SAID CELLULOSIC MATERIAL IN SAID DELIGNIFICATIONZONE OF SAID DIGESTER; COOLING SAID DELIGNIFIED A COOLING ZONE COMPRISEDOF AT LEAST TWO SUB-STAGES IN SAID DIGESTER; AND WITHDRAWING SAIDDILIGNIFIED MATERIAL FROM SAID DIGESTION ZONE.